The concentration of CH4 (C1) in 28 headspace samples analyzed from Site 1264 was at an atmospheric background level (range = 1.6–2.0 µL/L), and no hydrocarbon gases higher than C1 were detected.
Interstitial waters from 29 samples were collected at Site 1264: 28 from Hole 1264A (10.4–314.2 mcd) and 1 from Hole 1264B (5.9 mcd). Chemical constituents were determined according to the procedures outlined in "Geochemistry" in the "Explanatory Notes" chapter. Results of the chemical analyses are presented in Table T11.
The pH of pore waters at Site 1264 ranges from 7.3 to 7.6 (average = 7.4 ± 0.1) (Table T11). All values are lower than the average seawater value of 8.1, and the data exhibit no depth-dependent trend. Salinity values range from 34.5 to 35.5 g/kg.
Alkalinity decreases slightly from 2.6 mM in the shallowest sample at 5.9 mcd to 2.3 mM at 303.7 mcd (Fig. F24A). Two samples had values of 3.3 mM at 10.4 and 53.6 mcd.
The pore water chloride concentration generally increases with depth from a minimum value of 559 mM (5.9 mcd) to 573 mM (292.2 mcd) (Fig. F24B), although it shows significant variability about the mean of 565 mM (standard deviation = 5 mM). One sample contains a lower chloride value of 555 mM (212.6 mcd).
Sodium concentrations increase from 467 to 492 mM from 5.9 to 21.0 mcd and remain high (~485–500 mM) down to 107.5 mcd (Fig. F24C). The pore water sodium concentration decreases to 467 mM from 107.5 to 140.0 mcd. Below 140.0 mcd, the sodium concentration gradually increases to 483 mM at the base of the profile (314.2 mcd).
Site 1264 downhole trends in potassium, calcium, and magnesium are consistent with those resulting from exchange with basaltic basement at depth (Gieskes, 1981), with potassium and magnesium decreasing and calcium increasing slightly with depth (Fig. F24D, F24E, F24F). Pore water potassium concentrations decrease slightly from 11.2 mM (5.9 mcd) to 9.6 mM (314.2 mcd) (Fig. F24D). Calcium values generally increase from 11.4 mM (5.9 mcd) to 14.8 mM (314.2 mcd) (Fig. F24E). The magnesium profile (Fig. F24F) is characterized by a general decrease with depth from 55.6 mM in the shallowest sample (5.9 mcd) to 49.5 mM at the base of the section (314.2 mcd).
The strontium concentration increases from 101 µM (5.9 mcd) to 595 µM (169.5 mcd). Below this depth, strontium values remain high (>530 µM) down to the base of the analyzed record (314.2 mcd). The profile exhibits a slight decrease over the interval from 223.2 to 314.2 mcd (Fig. F24G). These high pore water concentrations may be related to carbonate diagenesis, in which dissolution of biogenic calcite and subsequent reprecipitation of inorganic diagenetic calcite supplies dissolved strontium to the interstitial waters (e.g., Baker et al., 1982).
The lithium concentration profile shows a decrease from 25.7 to 23.6 µM over the interval from 5.9 to 129.0 mcd then increases gradually to 30.9 µM at the base of the section (314.2 mcd) (Fig. F24H). This trend suggests a shallow subsurface sink for dissolved lithium and a deeper source of lithium from the sediment into the pore waters.
Boron values exhibit a general increase with depth from 453 µM (5.9 mcd) to 490 µM (314.2 mcd) (Fig. F24I). Pore water barium concentrations range between background values of <0.5 and ~2 µM (Fig. F24J). Elevated barium concentrations do not correlate with any decrease in pore water sulfate concentrations that might suggest enhanced barite solubility.
The pore water profile at Site 1264 is characterized by a general decrease in sulfate from 27.1 mM (10.4 mcd) to 24.6 mM (303.7 mcd) (Fig. F24K). Samples 208-1264B-1H-4, 140–150 cm (23.6 mM), and 208-1264A-9H-5, 140–150 cm (19.2 mM), yielded lower, and possibly spurious, values. The relatively high concentrations of sulfate (mean = 25.22 ± 0.77 mM) reflect the very low organic matter content of the sedimentary section recovered at Site 1264 (see "Carbonate and Organic Carbon" in "Sediment Geochemistry").
The manganese pore water profile exhibits low and relatively uniform values (less than ~1.0 µM) down to 140.0 mcd. Below this depth, values increase from 0.44 µM (140.0 mcd) to 3.34 µM (212.6 mcd) and increase further to 5.5 µM (303.7 mcd) toward the base of the profile (Fig. F24L).
The pore water concentration of dissolved iron is below detection limit throughout much of the interval analyzed (Fig. F24M). However, concentrations rise from 0 to 5.2 µM at 32.1 mcd and peak at 13.1 µM (118.8 mcd). Dissolved iron concentrations then decrease back to 0 µM at 160.0 mcd. Interestingly, maximum iron concentrations occur above the increase in dissolved manganese concentrations, and the maximum value corresponds to the division between lithostratigraphic Subunits IIA and IIB. This sedimentary interval contains abundant iron oxide "bands" that may have contributed dissolved iron to the pore waters (see "Lithostratigraphy").
Pore water silicon concentrations (Fig. F24N) decrease from the shallowest value of 339 µM (5.9 mcd) to 175 µM (86.8 mcd) then generally increase to 340 µM at the base of the profile (314.2 mcd). Zinc concentrations in pore waters from Site 1264 were low and variable, ranging from below detection to 1.4 µM (Fig. F24O).
The Site 1264 pore water profiles of potassium, calcium, and magnesium reflect the diffusional gradient between seawater and basalt. In contrast, the profiles of strontium, lithium, boron, and parts of the manganese and iron records are dominated by sedimentary contributions of dissolved ions to the pore waters. Little evidence of microbial influence exists in these profiles, as reflected in the sulfate, manganese, and iron profiles.
Carbonate determinations by coulometry were made for a total of 115 samples from Site 1264 (Table T12). Samples were selected to provide a measure of the carbonate content within different units and to assess the influence of carbonate content on color reflectance. The carbonate values in Unit I average 94.6 wt%. Below Unit I, carbonate contents remain relatively high but decrease gradually. Values in Unit II are slightly higher than those in Unit I (mean = 96.1 wt%) (Table T12; Fig. F25). Subunit IIB carbonate content averages 94.2 wt%, whereas the carbonate values of Subunits IIC and IID are slightly lower and more variable (mean = 92.3 and 93.0 wt%, respectively).
Elemental analysis of C indicates generally low concentrations of organic matter in Site 1264 sediments (Table T12). Several of the samples analyzed contained organic carbon concentrations slightly greater than zero, with values ranging from 0.0 to 0.3 wt%. None of the analyzed samples contained measurable nitrogen.
Extraction of organic matter (12 hr) was performed on Samples 208-1264A-1H-5, 145–150 cm; 9H-5, 140–150 cm; and 24H-5, 140–150 cm, after squeezing out the interstitial water.
The chromatogram for Sample 208-1264A-1H-5, 145–150 cm (10.4 mcd), was dominated by n-C14 through n-C18 alkanes with a minor component of branched isoprenoids (with the exception of pristane and phytane) (Fig. F26). The extracts from Samples 208-1264A-9H-5, 140–150 cm, and 24H-5, 140–150 cm (97.6 and 255.1 mcd, respectively), display a similar n-alkane abundance and distinct peaks of pristane and phytane; however, branched isoprenoids were more abundant than those in Sample 208-1264A-1H-5, 145–150 cm. The results indicate that the majority of sedimentary organic matter originates from photosynthetic algae.
A comparison of the Neogene samples from Site 1264 with the Paleogene samples from Site 1263 reveals a significant difference. Branched isoprenoids, especially anteiso-alkanes in Sample 208-1263A-17H-5, 140–150 cm, were dominant compounds in earlier eluates at m/z 85 (Fig. F26) on the mass chromatogram, suggesting a greater contribution of cyanobacterial products to the sedimentary organic matter. These results suggest a shift in major primary producers from cyanobacteria to photosynthetic algae between the middle Eocene and earliest Miocene.
A total of 55 squeeze cake samples (sediments remaining after interstitial water samples have been squeezed) from Sites 1263 and 1264 were analyzed shipboard by inductively coupled plasma–atomic emission spectroscopy following the method outlined in "Geochemistry" in the "Explanatory Notes" chapter. These analyses were intended to provide bulk carbonate chemical data, but the resulting calcium contents (Tables T13, T14) clearly demonstrate that phases other than carbonate have also been dissolved and analyzed (the average calcium content of the leachates is 51.1 wt% for Site 1263 and 49.7% for Site 1264, whereas the calcium content of pure CaCO3 is only 40 wt%). Therefore, these data (Tables T13, T14) are not useful for assessing the partitioning of elements between the pore waters and carbonates as intended but provide information about the chemical composition of acid-leachable components (carbonate plus some clay components of the sediment).
No lithium peaks were observed during analysis, suggesting that the 2000-fold diluted samples contained lithium at levels below the detection limit. Silicon concentrations in the leachates could not be determined because the silicon blank levels in the nitric acid dilution matrix used were too high.
Generally the sediment leachate data (Tables T13, T14) display no relationship with the corresponding pore water profiles with the exceptions of boron and strontium (discussed below). Calcium, potassium, and magnesium concentrations in Site 1263 leachates show no downhole trend and fluctuate between 54.4–49.6 wt%, 36.3–13.4 mM, and 77.7–43.9 mM, respectively. At Site 1264, leachate calcium, potassium, and magnesium concentrations also exhibit no downhole trend and fluctuate between 52.5–46.3 wt%, 25.5–9.3 mM, and 59.6–40.4 mM, respectively. The lack of a relationship between the calcium, potassium, and magnesium content of the leachates and the associated pore waters supports the interpretation that the clear downhole increase in pore water calcium and decrease in pore water potassium and magnesium at Sites 1263 and 1264 is the result of diffusion between seawater and basement basalt.
Iron concentrations in the leachates fluctuate between 55.5 and 15.3 mM in Site 1263 samples and between 50.3 and 17.9 mM in Site 1264 samples. There is no downhole trend or noticeable relationship with associated pore water iron concentrations for either Site 1263 or 1264.
Manganese concentrations in the leachates vary between 6.46–1.28 and 4.80–1.71 mM in Site 1263 and 1264 samples, respectively. No general downhole trend or relationship with pore water manganese is observed for either site.
Sodium concentrations in the leachates fluctuate between 193–66 and 225–129 mM in Site 1263 and 1264 samples, respectively. No downhole trend or relationship with pore water sodium is apparent at either site.
Barium concentrations in the leachates display no consistent downhole relationship with pore water barium in either Site 1263 or 1264 samples. In Site 1263 sediments, barium concentrations increase downhole from 0.92 mM at the top of the section (10.2 mcd) to peak at a value of 9.44 mM at 250.2 mcd before dropping to a value of 3.39 mM at the base of the section (378.2 mcd). In Site 1264 sediments, barium concentrations decrease downhole from 2.93 mM at 10.4 mcd to a minimum of 0.23 mM at 180.6 mcd before increasing to values >1 mM below 212.6 mcd.
Boron concentrations of leachates from Sites 1263 and 1264 follow the downhole fluctuations of pore water boron throughout the majority of both sections (Fig. F27). In the Site 1263 section, the sediment and pore water boron profiles show no relationship above ~75 mcd and below ~300 mcd, but throughout the interval between ~75 and ~300 mcd, the sediment and pore water profiles trace each other. The lack of a relationship between the sediment and pore water boron above ~75 mcd can be attributed to the downward diffusion of seawater into the sediments. Below 292.6 mcd, Site 1263 samples were recovered using the extended core barrel coring system, which could increase seawater contamination of the pore water samples (see "Geochemistry" in the "Site 1263" chapter). At Site 1264, all cores were recovered using the APC system, which produces the most undisturbed cores with minimum seawater contamination, and the sediment and pore water boron profiles trace each other down to the base of the section (314.2 mcd). However, in the upper ~100 m (especially the top 50 m) of the Site 1264 section, the sediment and pore water boron profiles exhibit little or no relationship, which is again suggestive of seawater diffusion into the sediments from above. Laboratory experiments under controlled temperatures and pressures have shown that boron is leached from terrigenous sediments into fluids (e.g., James et al., 2003), and a study of Leg 186 interstitial water samples concluded that the removal of boron from clays and volcanic ash was responsible for boron enrichment in the pore waters (Deyhle and Kopf, 2002). The strong relationship between the sediment and pore water boron profiles at Sites 1263 and 1264 supports the notion that the concentrations of boron in the pore waters are related to the amount of boron in the sediments. Interestingly, the pore water profiles of other elements (e.g., calcium, potassium, and magnesium) indicate that diffusion of dissolved species through the sediments has occurred, which would potentially mask the association of pore water boron to that of sediment boron concentrations through the smoothing of pore water boron concentration gradients. For the structure of the pore water boron profile to remain unaltered by such diffusion requires the ongoing release of boron from sediments to occur on timescales smaller than that at which diffusion is effective.
Strontium concentrations in the leachates of Site 1263 samples generally decrease downhole from 13.5 mM at the top of the section (10.2 mcd) to a minimum of 6.3 mM at 250.2 mcd before increasing downhole to values >13 mM near the base of the section (378.2 mcd) (Fig. F28). The Site 1263 pore water strontium profile increases downhole and peaks at values >250 µM below ~250 mcd and thus is inversely related to the sediment strontium profile to a depth of ~250 mcd. Such a relationship is explained by carbonate dissolution and recrystallization being focused around ~250 mcd, which releases strontium to the pore waters and produces inorganic calcite with substantially lower strontium concentrations than the original biogenic calcite (e.g., Baker et al., 1982). Below ~250 mcd, the strontium concentration of the leachates increases with no associated change in the strontium pore water profile. It is unlikely that carbonate dissolution and recrystallization has occurred at ~250 mcd and not in the sediments below. This is supported by the continuation of enhanced pore water strontium below ~250 mcd. Instead, the increase in the sediment strontium profile below ~250 mcd suggests a noncarbonate sedimentary strontium component, which does not release strontium to the pore waters and increases in abundance with depth below ~250 mcd, enhancing the strontium content of the sediments.
The sediment strontium concentrations in Site 1264 samples fluctuate downhole between 17.6 and 13.4 mM (Fig. F28) and exhibit no relationship with the associated pore water strontium profile. This is consistent with minimal carbonate dissolution and recrystallization in the Site 1264 section, as the sediment strontium values remain much higher (>13 mM) than those measured in the interval suspected to exhibit inorganic carbonate characteristics in Site 1263 sediments (6.3 mM). However, below ~100 mcd, the Site 1264 pore water strontium values are consistently more than twice the maximum values observed in the Site 1263 pore waters, suggesting that a noncarbonate sedimentary component rich in strontium is leaching strontium to the pore waters below ~100 mcd. These interpretations are not necessarily exclusive, as the influence of a strontium-rich noncarbonate sedimentary component could mask the signal of carbonate dissolution and recrystallization. Further work is required to assess the strontium chemistry of only the carbonate fraction and to diagnose the true extent of carbonate diagenesis in the sediments of Sites 1263 and 1264.